The invention relates to a process for ultrasound treatment of a strip of material, and to a device for implementing the process.
An ultrasound treatment system for treatment of a strip of material, in which the strip of material passes through a treatment gap, which is formed by a roller device that includes a rotating roller body and a sonotrode directed approximately radially against the working surface of the roller body, is known from German Patent 195 13 246 C2. In such systems, special requirements concerning the concentricity of the roller are set, in order to ensure uniform quality of the strip of material after the treatment.
This is particularly true for ultrasound bonding of nonwoven fabrics made of thermoplastic fibers. Here, the ultrasound generator is radially placed against the counter-roller via a sonotrode, i.e. the transmitter of the ultrasound, and the counter-roller generally bears a bonding pattern in the form of a surface engraving. At the raised locations of the surface engraving, the transfer of ultrasound energy to the nonwoven fabric is particularly intensive, so that here, a preferential temperature increase and preferential melting of the fibers occur, and in this manner, bonding of the individual filaments or fibers of the nonwoven fabric, following a pattern, at certain points or along certain lines, takes place, thereby bonding the layer of nonwoven fabric in and of itself. Furthermore, laminates, i.e. laminate materials made up of films and nonwoven fabrics, can also be produced in this manner.
Preferably, the sonotrode is rigidly connected with the roller device, since this makes it possible to introduce a maximum amount of energy into the strip of material passing through. xe2x80x9cRigidlyxe2x80x9d means that the sonotrode is affixed to the machine frame, which also carries the roller device, without the insertion of any resilient element.
When using such devices for ultrasound treatment of a strip of material, it has been shown, in the past, that at first, very good treatment results are obtained. However, after the speed or the amount of energy introduced is increased, the work result deteriorates, because the roller body of the roller device starts to bend. This effect particularly occurs if there are fundamental concentricity defects. Any bending that has come about will cause the gap that is formed between the working surface of the roller body and the sonotrode to become larger and smaller with the cycle of rotation of the roller body. When the gap is smaller, more energy flows into the bonding process, and the roller body will become warmer at this location. This will cause it to expand further, resulting in further reinforcement of the bending effect, until this bending has reached such an extent that ultrasound treatment becomes impossible.
The invention is therefore based on the task of developing a process and a device for ultrasound treatment of a strip of material, in which any bending of the roller body is at least significantly reduced, even when a high amount of energy is introduced by the sonotrode, and, particularly, even if the sonotrode is rigidly connected with the roller device.
This task is accomplished with a process for ultrasound treatment of a strip of material, comprising:
passing the strip of material through a treatment gap, the gap formed by a roller device that includes a rotating roller body and a sonotrode directed approximately radially against a working surface of the roller body; and
cooling the roller body, approximately uniformly over its circumference, directly below the working surface of the roller body.
The task is further accomplished with a device for ultrasound treatment of a strip material, comprising:
a roller device that includes a rotating roller body, and a treatment gap formed by a sonotrode directed approximately radially against a working surface of the roller body, wherein a cooling device for cooling the roller body is provided directly below the working surface of the roller body.
Surprisingly it has been shown that the concentricity defect can be significantly reduced, even if introduction of energy by the sonotrode is at a high level, and can regularly be reduced to values as low as  less than 5 xcexcm, if the roller body is cooled directly below its working surface. xe2x80x9cDirectlyxe2x80x9d in this connection is to be understood to mean that the roller mantle has a wall thickness, in the direction towards the axis of rotation of the roller, that is at least twice, preferably at least five times as great as the wall thickness of the cooled, outside region of the roller body that forms the working surface. The improvement in the concentricity defect when using the process according to the present invention can be explained by the fact that the heat introduced is conducted away directly below the working surface, so that the remainder of the material of the roller cross-section remains at a uniform temperature even at angle positions in which a greater amount of energy is introduced, and therefore counteracts the bending on the basis of thermal, unilateral expansion. Since, according to the present invention, the region at a uniform temperature has a greater wall thickness than the outer region that might be heated more strongly locally, it counteracts any bending of the roller body.
Cooling of the roller body directly below the working surface preferably takes place in that a fluid cooling medium is passed through cooling channels that are distributed uniformly over the circumference of the roller body and arranged directly below the working surface of the roller body.
It is particularly preferred if the fluid cooling medium is passed through lengthwise bores that run directly below the working surface, approximately parallel to the latter.
In order to prevent the cooling medium from flowing along a preferred path, characterized by the lowest flow resistance, due to different flow cross-sections, for example, the cooling medium is preferably passed through the cooling channels at such a speed that turbulent flow prevails in them.
Experiments have shown that the best treatment results are achieved if the roller body is temperature-controlled in its interior, i.e. in its region that extends radially in the direction towards the axis of rotation of the roller body, away from the cooling means, and, preferably, if the temperature difference between the working surface of the roller body and the cooling medium is a maximum of 2xc2x0 C.
The device according to the present invention, for ultrasound treatment of a strip of material according to the process according to the present invention, therefore has means for cooling the roller body located directly below the working surface of the roller body.
These are preferably formed by cooling channels that are distributed uniformly over the circumference of the roller body and arranged directly below the working surface of the roller body.
The means for cooling are comparatively simple to produce and very effective if they are made up of lengthwise bores that are arranged approximately parallel to the working surface of the roller body.
They are preferably structured in such a way, for reasons already indicated above, that when the cooling medium flows through them, turbulent flow prevails in them.
The roller body of the roller arrangement furthermore preferably has means for temperature control of the roller body, which keep the region that lies between the means for cooling the roller surface and the axis of rotation of the roller body at a uniform temperature. The means for temperature control can be electrically operated heating means, for example electric heating rods.
Because the amount of energy that can be introduced into the strip of material that passes through the roller gap in this way is so high, it is especially preferred if the sonotrode is rigidly connected with the roller device, i.e. without the insertion of any elastic elements.